Electrostatic shield for high voltage cables



Allg 28, 1962 E. c. LEE 3,051,771

ELEcTRosTATIc SHIELD EOE HIGH VOLTAGE CABLES Filed July 29, 1959 Inventor E G EE www Attorney Patented Aug. 28, 1962 3,051,771 ELECTROSTATIC SHELD FOR HIGH VOLTAGE CABLES Edwin Charles Lee, London, England, assigner to International Standard Electric Corporation, New York,

N.Y., a corporation of Delaware Filed July 29, 1959, Ser. No. 830,236 Claims priority, application Great Britain Aug. 15, 1958 3 Claims. (Cl. 174-36) The present invention relates to electric cables and more particularly to means for screening the cores of high voltage power cables.

In electric cables it is necessary to ensure that the dielectric surrounding the conductor is subjected only to radial electric stress and for this purpose it is known to apply a conducting screen to the outside of the dielectric. The presence of such a screen gives rise to a further diiculty, however, in that voltages and associated currents will be induced in it by the currents which flow in the conductor itself. For low voltage applications the effect is not important, since the induced voltages and currents are correspondingly small. At higher operating voltages, however, they may be large enough to cause overheating of the dielectric and eventual breakdown and must be eliminated or greatly reduced.

In order to limit the voltages which may be induced along the length of the screen it is necessary to ensure that it has a relatively low longitudinal resistance, and for this purpose it is usual to ensure that the screen makes Contact at frequent intervals with the lead sheath of the cable.

Known forms of screening material such as paper which has a metal foil attached to one surface, or carbon black paper interlocated with a metal tape, suffer from the disadvantage that they are easily damaged during manufacture, leaving parts of the dielectric unscreened.

In the case of three-core cables where a binder is required to restrain the cores this binder may be provided with metal wires which ensure contact between the in dividual core screens and the lead sheath. The binder also serves to protect the screens against damage during manufacturing operations. Recent development of 3- core oil filled cables without fillers or duct spirals precludes the provision of such an external binder tape, however, since this adds considerably to the longitudinal fluid resistance. Also, although such cables may be made with a temporary paper which is removed immediately prior to lead sheathing there is considerable risk of damage to the screen due to the cores spreading during the lead sheathing operation.

Accordingly the present invention provides an electric cable in which the screen surrounding a core comprises paper covered on both sides by a metallic layer and arranged in such manner that the entire surface of said core is covered by at least one thickness of said paper.

The invention will be described With reference to the accompanying drawings in which:

FIG. l shows a longitudinal section through the outer dielectric paper and the screen in a known arrangement;

FIG. 2 shows a longitudinal section through a further known arrangement;

FIG. 3 shows a longitudinal section through the outer dielectric paper and the screen according to the present invention; and

FIG. 4 shows an alternative form of metallised paper used in the arrangement according to the invention.

In the known arrangement illustrated in FIG. 1 the outer dielectric paper 1 is wound in a gapped manner and covered by the metallised paper 2, which has its metallic layer designated 3. Metallised paper 2 is wound with an overlap to ensure that the dielectric is completely covered. It is perforated to permit easy drying and impregnation but if used without a protective covering it may be damaged during manufacture and leave portions of the surface unscreened.

The known arrangement shown in FIG. 2 consists of alternate overlapping windings of carbon black paper 4 and metal tape 5. The outer layer of the dielectric is again designated 1. If the metal tape is damaged the carbon paper will normally provide containuity of screening surface, though it is still possible to damage it Iso much that portions of the surface are left unscreened. A further disadvantage is that the metal tape does not contract on drying as does the dielectric so that it often happens that the metal tape becomes loose. If this happens the screen is not completely effective, since any spaces beneath the screen are a source of weakness. Finally, it is believed that the use of the carbon paper can contribute losses to 'the cable.

In FIG. 3 the screen consists of an overlapping, winding of paper 6 which has metallised foils 7 and S secured to its surfaces. In this case not only is effective continuity obtained between individual turns but, also, should the outer foil 8 be damaged then there always remains the inner foil '7 which guarantees complete coverage of the surface. It has been found that such damage as occurs during manufacture usually affects the outer foil 8 and not the paper 6 to which it is attached.

The arrangement illustrated in FIG. 3 avoids the use of expensive carbon black paper and also the danger due to differential contraction during drying. The paper, 6, and foils, 7 and S, may be perforated in the usual manner to permit of drying and impregnation.

An alternative arrangement to that illustrated in FIG. 3 uses the paper and foil shown in FIG. 4. The paper 9 now has metal foil secured to one surface only, 10, but is equivalent to a layer of paper metallised on both surfaces when folded over in the manner shown. The double thickness is applied with an overlap, as in FIG. 3.

Metallised paper of the type illustrated in FIGS. 3 and 4 and applied in the manner illustrated in FIG. 3 may be used to screen the individual cores of high voltage 3-core cables when it is not possible to use an external binding tape.

While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. The combination of an electric cable and an electrostatic screen surrounding the cable core, said screen comprising paper covered on both sides with a metallic layer and arranged in such manner that the entire surface of rsaid core is covered by at least two metallic layers.

2. The combination according t-o claim l in which said screen is applied to said core in the form of a helical winding, successive turns of said winding overlapping.

3. An electric cable comprising a core and an elec- 3 4 trostatic screen helically Wound on said core, said elec- References Cited in the file of this patent trostatic screen comprising a strip of paper rnetallized UNITED STATES PATENTS on one side and folded over so that the metallized layer 4 ,6 t 34 1s on the outs1de of the fold and the ends of the fold are g4; lshe al Egg 34 c-oeXtensive, whereby the folded paper has a U-shaped 5 1983520 Chamb gt- Dec: 11 1934 cross section having two layers of metal joined at one 2:469416 Smyer May 10i 1949 end, successive turns of said screen on said core overlapping in such a manner that the entire surface of said FOREIGN PATENTS core is covered by at least two layers of metal. 1,186,769 France Feb. 23, 1959 

